Cracking tube having helical fins

ABSTRACT

A cracking tube ( 50 ) for use in thermal cracking furnaces for producing ethylene or the like has fins ( 1 ) formed on an inner surface thereof and inclined with respect to an axis of the tube for stirring a fluid inside the tube. The fins are arranged discretely on one or a plurality of helical loci, and the tube inner surface has regions (Z B ) wherein no fins are present over the entire axial length of the tube from one axial end of the tube to the other axial end thereof.

TECHNICAL FIELD

The present invention relates to cracking tubes for use in thermalcracking reactors for producing ethylene or the like, and moreparticularly to a cracking tube which is provided on the inner surfacethereof with fins for stirring the fluid therein and which is adapted tosuppress pressure losses to the greatest possible extent whileeffectively promoting heat transfer to the fluid therein.

BACKGROUND ART

Olefins such as ethylene, propylene or the like are produced bythermally cracking material gases of hydrocarbons (naphtha, natural gas,ethane, etc.). The thermal cracking reaction is conducted by introducingthe hydrocarbon material gas and steam into a cracking coil disposedwithin a heating furnace supplied with heat from outside, and heatingthe mixture to a reaction temperature range while the mixture flowsthrough the coil at a high velocity.

Typically, the cracking coil comprises a plurality of (straight) tubeswhich are connected into a zigzag assembly by bends.

To conduct the thermal cracking reaction efficiently, it is important toheat the fluid flowing inside the coil at a high velocity to thereaction temperature range radially inward to the central portion of thetube channel within a short period of time and to avoid heating at ahigh temperature to the greatest possible extent. If the gas is heatedat a high temperature over a prolonged period of time, lighter fractionsof hydrocarbons (methane, free carbons, etc.) will be produced inexcessive amounts or the product of cracking will undergo, for example,a polycondensation reaction to reduce the yield of the desired product.Promoted coking (deposition of free carbon on the tube inner wall) willalso result to lower the coefficient of heat transfer, giving rise to aneed to perform decoking frequently.

Accordingly it is practice to provide fins on the tube inner surface ofthe cracking coil as elements for stirring the fluid within the tubes.The fluid flowing at a high velocity produces turbulence by beingstirred by the fins, and can be heated to a higher temperature rapidly.As a result, the reaction is completed within a shortened period oftime, while production of lighter fractions due to excessive cracking isavoided. Furthermore, an improvement in the coefficient of heat transferof the tubes makes it possible to lower the temperature of the tubes,producing an effect to improve the serviceable life of the tubes.

FIGS. 12 to 14 show in development proposed examples of fins on crackingtubes (JP-A No. 1997-241781).

FIG. 12 shows fins 1 continuously extending helically at a constantangle of inclination with the tube axis.

FIG. 13 corresponds to the continuous helical fins of FIG. 12 as formeddiscretely. Fins 1 and nonfin portions 2 on helical loci are in astaggered arrangement wherein the fins are replaced by nonfin portionsevery turn of helix.

These examples have a great effect to stir the fluid within the tubesand are highly efficient in heat transfer to the fluid within the tubes,whereas the internal pressure of the fluid inside the tubes builds upowing to a great pressure loss of the fluid, entailing the drawback thatthe cracking operation produces ethylene, propylene or the like in alower yield.

FIG. 14 shows fins 1 and nonfin portions 2 arranged alternately on aplurality of lines parallel to the tube axis. However, the finspositioned in parallel to the tube axis fail to produce a sufficienteffect to stir the fluid inside the tubes and to achieve the desiredheat transfer performance.

In view of the above problems, an object of the present invention is tosuppress pressure losses to the greatest possible extent whilemaintaining an effect to promote heat transfer to the fluid within thetube.

SUMMARY OF THE INVENTION

To fulfill the above object, the present invention provides a crackingtube which has fins formed on an inner surface thereof and inclined withrespect to an axis of the tube for stirring a fluid inside the tube, thefins being discretely arranged on one or a plurality of helical loci,the tube inner surface having regions wherein no fins are present overthe entire axial length of the tube from one axial end of the tube tothe other axial end thereof.

The tube of this construction is adapted to minimize the pressure lossof the fluid inside the tube while permitting the helical fins totransfer heat to the inside fluid with a high efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a development of the inner surface of a cracking tube of theinvention for illustrating an embodiment of arrangement pattern of finsformed on the tube inner surface.

FIG. 2 is a development of the inner surface of a cracking tube of theinvention for illustrating another embodiment of arrangement pattern offins formed on the tube inner surface.

FIG. 3 is a development of the inner surface of a cracking tube of theinvention for illustrating another embodiment of arrangement pattern offins formed on the tube inner surface.

FIG. 4 is a development of the inner surface of a cracking tube of theinvention for illustrating another embodiment of arrangement pattern offins formed on the tube inner surface.

FIG. 5 is a development of the inner surface of a cracking tube of theinvention for illustrating another embodiment of arrangement pattern offins formed on the tube inner surface.

FIG. 6 is a diagram for illustrating the arrangement of fins shown inFIG. 3.

FIG. 7 is a view in cross section and showing the tube of the embodimentshown in FIG. 1.

FIG. 8 is a diagram for illustrating an overlaying method of forminghelical fins in the form of overlaid beads.

FIG. 9 is a graph showing heat transfer characteristics of test tubesdetermined by the experiment.

FIG. 10 is a graph showing pressure loss characteristics of the testtubes determined by the experiment.

FIG. 11 is a diagram for generally illustrating the configuration of atest coil.

FIG. 12 is a development of the inner surface of a conventional crackingtube for illustrating a pattern of fin formed on the tube inner surface.

FIG. 13 is a development of the inner surface of a conventional crackingtube for illustrating another arrangement pattern of fins formed on thetube inner surface.

FIG. 14 is a development of the inner surface of a conventional crackingtube for illustrating another arrangement pattern of fins formed on thetube inner surface.

BEST MODE OF CARRYING OUT THE INVENTION

The cracking tube of the present invention will be described below indetail with reference to the illustrated embodiments.

FIG. 1 is a development of the inner surface of a tube showing anembodiment of arrangement of helical fins according to the presentinvention.

Fins 1 are formed discretely along a helical locus which is positionedat a predetermined angle of inclination θ with respect to the axialdirection x of the tube. The helical locus is indicated by slantingdotted lines, and connections of the helix are indicated by verticaldotted lines. Horizontal chain lines show regions Z_(A) wherein fins arearranged in the axial direction, and regions Z_(B) of nonfin portions 2wherein no fins are present.

In the embodiment of FIG. 1, four fins are arranged along every turn ofhelix. The corresponding fins 1, as well as the corresponding nonfinportions 2, on helical lines representing respective turns of helix arearranged in a direction parallel to the tube axis.

FIGS. 2 to 5 are developments of the inner surfaces of tubes showingother embodiments of arrangements of helical fins according to theinvention.

FIG. 2 shows helical fins formed along continuous helical loci which aredifferent in the angle of inclination θ. The angle of inclination θ I ofthe helix in a region I of the tube channel is larger than the angle ofinclination θ II of the helix in a region II thereof. The fins 1 andnonfin portions 2 are arranged within the respective regions Z_(A) andZ_(B) which are parallel to the tube axis.

FIG. 3 shows an embodiment wherein two helical loci are provided. Finsare formed discretely at the same angle of inclination θ alongrespective helical loci S1 and S2. Fins 11 and nonfin portions 21 areformed on the helical locus S1, fins 12 and nonfin portions 22 areformed on the helical locus S2, the fins 11, 12 are arranged withinregions Z_(A), and the nonfin portions 21, 22 are arranged withinregions Z_(B).

FIG. 4 shows an embodiment wherein helical fins are formed along twohelical loci S1 and S2, and fins along the locus S1 are different fromthose along the locus S2 in size. The fins 11 along the helical locus S1are longer than the fins 12 along the helical locus S2. The fins 11, 12are arranged within regions Z_(A), and all or some of nonfin portions21, 22 are arranged within regions Z_(B).

FIG. 5 shows an embodiment wherein helical fins are formed along fourhelical loci S1 to S4, and fins 11 to 14 along the respective helicalloci S1 to S4 are arranged at slightly varying intervalscircumferentially of the tube. The group of fins 11 to 14 on therespective four helical loci S1 to S4 are arranged within a regionZ_(A), and a group of nonfin portions 21 to 24 on these loci S1 to S4are arranged within a region Z_(B). These fins 11 to 14 within theregion Z_(A) are arranged along a wave (indicated in a chain line).

Thus, according to all the embodiments of FIGS. 2 to 5, the tube innersurface has regions Z_(B) wherein no fins are present over the entireaxial length of the tube from one axial end of the tube to the otheraxial end thereof.

FIG. 6 is a diagram for illustrating the arrangement of fins shown inFIG. 3. Indicated at θ is the angle of inclination of the helical fins,and at p is the fin pitch that is the center-to-center distance betweencorresponding fins on the adjacent helical lines in the direction oftube axis. These values are determined suitably according to the insidediameter D of the tube.

In the case of a tube having an inside diameter D of about 30 to 150 mm,for example, the angle of inclination θ can be about 15 to about 85degrees, and the pitch p, about 20 to 400 mm. The pitch p is increasedor decreased for adjustment depending on the angle of inclination θ ofthe helix and the number N of helixes (p=E/N wherein E is helix lead).

The height H (the height of projection from the tube inner surface) ofthe fins is, for example, about one-thirtieth to one-tenth of the insidediameter of the tube. The length L of the fins is, for example, about 5to 100 mm, and is determined, for example, according to the insidediameter D of the tube and the number of divided fins along every turnof helical locus.

FIG. 7 is a sectional view of helical fins in a plane orthogonal to theaxis of the tube, and shows an embodiment wherein four fins are arrangedon one turn of helical line. Suppose the fin has a circular arc length(as projected on a plane) w and the number of fins on one turn ofhelical line is n. The total circular arc length TW of the fins is thenTW=w×n.

Incidentally, the proportion of the total circular arc length TW of thefins to the circumferential length C(C=πD) of the tube inner surface,namely, R(R=TW/C), is preferably about 0.3 to 0.8 in order to ensure aminimized pressure loss while permitting the helical fins to promoteheat transfer to the fluid inside the tube. If this value is too small,the effect to promote heat transfer will be lower, whereas if the valueis excessively great, an excessive pressure loss will result.

The helical fins can be efficiently formed as beads by an overlayingmethod such as plasma powder welding (PTA welding). FIG. 8 shows anexample of welding operation.

A tube 50 is horizontally supported by a rotary drive apparatus (notshown) and rotatable about its axis x. A welding torch 51 is fixed to asupport arm 52, which is held parallel to the tube axis and is movableforward or rearward axially of the tube.

A powder (material for overlaying) is supplied by a pipe 53 to thewelding torch 51, which forms beads on the tube inner surface. Plasmawelding is performed intermittently by the rotation of the tube 50 andthe horizontal movement (in the direction of the tube axis) of thewelding torch 51 to form helical fins comprising beads formed byoverlaying.

In the case where two welding torches 51 are installed as illustrated,fins are formed along two helical loci.

The number of helixes of fins to be formed, the angle of inclination θ,pitch p, the number and width (circular arc length of projected image inFIG. 6) of fin regions Z_(A), etc. are adjustable suitably by varyingthe speed of rotation of the tube 50, the number of welding torches 51installed, the speed of horizontal movement thereof, the cycle ofintermittent application of the plasma arc, etc.

Helical fins are arranged over the entire length of tube channel fromthe inlet end of the tube to the outlet end thereof, or at suitableportion or portions of the channel, for example, in at least one of aregion in the vicinity of inlet end of the channel, intermediate regionthereof and a region in the vicinity of the outlet end.

The material for forming the helical fins is the same kind ofheat-resistant alloy as the tube, such as 25Cr—Ni(SCH22),25Cr-35Ni(SCH24) or Incoloy(Trademark) Also suitably usable are otherheat-resistant alloys which are serviceable in the environment whereinthe tube is to be used.

The present invention will be further described with reference tospecific examples.

EXAMPLE 1

Test Tubes T1 to T5 were prepared and checked for film heat transfercoefficient h (W/m²/K) and pressure loss dP (Pa).

T1 is according to the invention, and T2 to T5 are comparative examples.Table 1 shows the particulars about these test tubes. TABLE 1 Test TubesFin Specifications Circum- Circular Number ferential arc Number of rowsAngle length length Number per in circum- of Length of inner Draw-Thick- as pro- of turn of ferential incli- ratio No. I.D. surface ingShape Height ness jected helixes helix direction nation Pitch R * NoteT1 42 mm 132 mm FIG. Dis- 2.2 mm 8 mm 16.5 mm 1 4 — 60 deg 76 mm 0.5Inven- 1 crete, tion helical T2 42 mm 132 mm FIG. Con- 2.2 mm 8 mm 18.9mm 1 — — 60 deg 76 mm 1.0 Comp. 12 tinuous, ex. helical T3 42 mm 132 mmFIG. Dis- 2.2 mm 8 mm 18.9 mm 1 4 and 3 — 60 deg 76 mm 1.0 Comp. 13crete, alter- ex. helical nating (stag- gered) T4 42 mm 132 mm FIG.Paral- 2.2 mm 8 mm 18.9 mm — — 8 — — 0.5 Comp. 14 lel to ** ex. tubeaxis T5 42 mm 132 mm None No fin — — — — — — — — — Comp. ex.(Note)* Circular arc length ratio R = (sum of circular arc lengths of finsalong every turn of helix, as projected on a plane orthogonal to tubeaxis)/(circumferential length of tube inner surface)** The circular arc length ratio for T4 was determined as (sum ofthicknesses of fins)/(circumferential length of tube inner surface).

Experimental conditions are as follows.

Test fluid: air

Fluid temperature (inlet end): room temperature

Reynolds number: 20,000-60,000

Pressure loss measuring section: 1000 mm

The results of measurement are shown in FIG. 9 (film heat transfercoefficient h) and FIG. 10 (pressure loss dP). Each measurement is shownrelative to the value of Test Tube T5 at a Reynolds number of 20,000which value is taken as 1.0 (reference value).

FIGS. 9 and 10 reveal that Test Tube T1 of the invention is comparableto Test Tube T2 having a continuous helical fin and Test Tube T3 havingdiscrete helical fins in heat transfer characteristics and is comparableto Test Tube T4 in pressure loss.

However, Test Tubes T2 and T3 are greater than Test Tube 1 in pressureloss and result in a lower yield as will be described later.

On the other hand, Test Tube T4 is inferior to Test Tube T1 in heattransfer characteristics and therefore has the problem of permittingcoking in addition to a lower yield.

Test Tube T5 is a smooth-surfaced tube having no fins and accordinglysuperior to Test Tube T1 of the invention with respect to pressure loss,but is inferior in heat transfer characteristics and involves theproblem of yield and coking like Test Tube T4.

In contrast, Test Tube T1 of the invention is adapted to ensure aminimized pressure loss while maintaining the desired heat transfercharacteristics.

EXAMPLE 2

Next, a thermal fluid analysis was conducted using a W-shaped coil shownin FIG. 11 and simulating the conditions under which reactors are usedfor producing ethylene to determine pressure loss of the fluid insidethe coil and yields of ethylene and propylene.

The coil shown in FIG. 11 includes tubes (straight tubular portions)which are 63.5 mm in inside diameter, 6.4 mm in wall thickness and 9.6 min length and which provide a first pass, second pass, third pass andfourth pass, respectively, as arranged in this order from the upstreamside downstream. Table 2 shows the construction of Test Tubes T6 to T9.

Test Tube T6 is according to the invention, and Test Tubes T7 to T9 arecomparative examples. As to the arrangement of fins on the tube(straight tubular portion), Test Tube T6 is the same as is shown in FIG.1, T7 as is shown in FIG. 13, and T8 as is shown in FIG. 12. T9 is anexample which has no fins. TABLE 2 Test Tubes Structure of T 6 T 7 T 8 T9 passes (Invention) (Comp. ex.) (Comp. ex.) (Comp. ex.) First pass Nofin No fin No fin No fin Second pass No fin No fin No fin No fin Thirdpass Fin A* Fin B** Fin C*** No fin Fourth pass Fin A* Fin B** Fin C***No fin(Note)*Fins A: Discrete helical fins (4 fins/turn of helix) in the arrangementof FIG. 1, 60 deg in angle of inclination, 3.5 mm in the height of fins,115.2 mm in pitch.**Fins B: Discrete helical fins in the arrangement of FIG. 13, 60 deg inangle of inclination, 3.5 mm in the height of fins, 115.2 mm in pitch.***Fin C: Continuous helical fin extending as shown in FIG. 12, 60 degin angle of inclination, 3.5 mm in the height of fin, 115.2 mm in pitch.

The analysis conditions are fluid pressure at the coil outlet of 1.98kg/cm (absolute pressure), coil inlet temperature of 600° C. and coiloutlet temperature of 830° C. Naphtha was caused to flow through onecoil at a flow rate of 840 kg/h, and steam at a flow rate of 420 kg/h.

Table 3 shows the temperature of the first to fourth passes of the coil.

Table 4 shows the results of analysis, i.e. the pressure and temperatureat the coil inlet and outlet, pressure loss and ethylene and propyleneyields. TABLE 3 Temperature at each Test tubes pass of T 6 T 7 T 8 T 9test tubes (Invention) (Comp. ex.) (Comp. ex.) (Comp. ex.) First pass849 847 846 860 (° C.) Second pass 870 868 867 881 (° C.) Third pass 880879 877 906 (° C.) Fourth pass 915 914 913 936 (° C.)

TABLE 4 Test tubes T 6 T 7 T 8 T 9 (Invention) (Comp. ex.) (Comp. ex.)(Comp. ex.) Coil inlet pressure (kg/cm²)* 3.68 3.84 3.98 3.25 Coiloutlet pressure (kg/cm²)* 1.98 1.98 1.98 1.98 Pressure loss of fluid(kg/cm²)* 1.70 1.86 2.00 1.27 Coil inlet temperature (° C.) 600 600 600600 Coil outlet temperature (° C.) 830 830 830 830 Ethylene yield (wt %)26.8 26.5 26.3 26.1 Propylene yield (wt %) 16.5 16.2 16.1 15.9(Note)*Absolute pressure

Table 3 reveals that T6 is comparable to T7 and T8 in tube temperatureand about 20 C lower than T9. This means that T6 to T8 are comparable inheat transfer efficiency and can be operated at a lower temperature.

Table 4 shows that T6 is smaller than T7 and T8 in pressure loss andexcellent in ethylene and propylene yields. Although small in pressureloss, T9 is inferior in heat transfer efficiency and therefore lower inethylene and propylene yields.

INDUSTRIAL APPLICABILITY

The arrangement of helical fins formed on the inner surface of thecracking tube of the invention enables the tube to minimize the pressureloss of the fluid inside the tube while permitting the tube to maintainhigh heat transfer characteristics due to the stirring action of fins.Accordingly the tube achieves improved ethylene and propylene yields,diminishes decoking work for the tube, has a prolonged life and istherefore useful as a cracking tube for thermal cracking furnaces forproducing ethylene or the like.

1. A cracking tube having fins formed on an inner surface thereof andinclined with respect to an axis of the tube for stirring a fluid insidethe tube, the cracking tube being characterized in that the fins arearranged discretely on one or a plurality of helical loci, the tubeinner surface having regions wherein no fins are present over the entireaxial length of the tube from one axial end of the tube to the otheraxial end thereof.
 2. The cracking tube according to claim 1 wherein thefins have an angle of inclination of 15 to 85 degrees.
 3. The crackingtube according to claim 1 wherein assuming that the sum of the circulararc length of fins is TW (TW=w×n wherein w is the circular arc length ofthe fin as projected on a plane orthogonal to an axis of the tube, and nis the number of fins on one turn of the helical locus), and that thecircumferential length of the tube inner surface is C(C=πD wherein D isthe inside diameter of the tube), the ratio TW/C is 0.3 to 0.8.
 4. Thecracking tube according to claim 1 wherein the fins are weld beadsformed by overlaying.
 5. The cracking tube according to claim 2 whereinassuming that the sum of the circular arc length of fins is TW (TW=w×nwherein w is the circular arc length of the fin as projected on a planeorthogonal to an axis of the tube, and n is the number of fins on oneturn of the helical locus), and that the circumferential length of thetube inner surface is C(C=πD wherein D is the inside diameter of thetube), the ratio TW/C is 0.3 to 0.8.